Study On Dynamic Detection Of Beam Structures Based On Local Vibration

For large-scale structures, it is very difficult to simultaneously identify the location and extent of element-level damage by using of the limited modal measurements, which often leads to wrong results of damage identification. The methodology, which is composed by the global identification and local identification, is an effective approach to identify the structural damage in complex structures. In the dissertation the local damage identification of beam-like structures is studied with modal measurements based on local vibration. The main contents of the dissertation are listed as follows.Firstly, the differential equations of lateral bending free vibration of Eular-Bornoulli model and Timoshenke model are built. Frequency equations of two different beams are then derived with elastic rotation constraints at supports. According to the given boundary conditions, the natural frequencies of the beams are obtained.Secondly, the method to identify the flexural stiffness and boundary conditions with the frequencies is proposed. The feasibility to identify the stiffness and boundary conditions simultaneously is discussed. The accuracy of the proposed method and the effects of measurement errors are also studied. It can be concluded that at least 3 frequencies are needed to identify the flexural stiffness and boundary conditions in the case of beam with different constraints at supports, and at least 2 frequencies are needed in the case of beam with same constraints at supports. The numerical simulations also show that the identification resufts of flexural stiffness have high accuracy than those of boundary constraints, which requires the more accurate measurements of frequencies. It is noted that identification accuracy of the beam with strong constraints is more dependent on measurement accuracy, especially in the case of the high frequencies.Finally, local damage localization based on dynamic signatures is presented. The applicability of three dynamic signatures based frequency data are studied, and the numerical simulations of different damage cases are carried out. The results demonstrate that the normalized change ratio of frequency is stable as changing the damage location and extent. Therefore, the normalized change ratio of frequency is selected as dynamic signature of beam-like structures based on local vibration. The effectiveness of the damage localization by using of the proposed dynamic signature is illustrated by a numerical example.